Reset InstallXLogFileSegmentActive after walreceiver self-initiated exit.

Reset InstallXLogFileSegmentActive after walreceiver self-initiated exit.

After commit cc2c7d65fc27e877c9f407587b0b92d46cd6dd16 added this flag,
failure to reset it caused assertion failures.  In non-assert builds, it
made the system fail to achieve the objectives listed in that commit;
chiefly, we might emit a spurious log message.  Back-patch to v15, where
that commit first appeared.

Bharath Rupireddy and Kyotaro Horiguchi.  Reviewed by Dilip Kumar,
Nathan Bossart and Michael Paquier.  Reported by Dilip Kumar.

Discussion: https://postgr.es/m/CAFiTN-sE3ry=ycMPVtC+Djw4Fd7gbUGVv_qqw6qfzp=JLvqT3g@mail.gmail.com